Murine cytomegalovirus (MCMV), the mouse homolog of human CMV, is a well- characterized model system for the study of CMV infection in a mammalian host. There is a growing interest in the anti-viral role of MHC class Ib-restricted T cells, referred to as non- classical T cells. During the previous funding period, using mice deficient in conventional CD8+ T cells or CD1d-restricted NKT cells, we showed that non-classical T cells contribute significantly to the immune response to MCMV infection. Our preliminary data also demonstrate that a subset of non-classical CD8+ T cells, likely restricted by the Qa-1 molecule, is sufficient to protect against MCMV induced lethality. We therefore hypothesized that non-classical T cells may substitute for NK cells and conventional T cells when their effector responses are impaired during viral immuno-evasion. The objective of this proposal is to understand how non-classical T cells contribute to the immune response to viral infections. To accomplish these goals, we generated and acquired a variety of unique tools and animal models. These include KbDbQa-1 deficient animals, a new J?18-/- mouse line, an MCMV open reading frame library, and a large number of MCMV mutants. In Specific Aim 1, we will define MCMV specific Qa-1 restricted CD8+ T cells and identify the MCMV peptide(s) they recognize. In Specific Aim 2, we propose to investigate the role of CD1d, Qa-2 and M3 restricted T cells during MCMV infection. This proposal fills a major gap in our understanding of the role of non-classical T cells during viral infection. The results from these studies are likely to have relevance to infections caused by other viruses and aid in the design of strategies directed at manipulating non-classical T cells.